1. Field of the Invention
The present invention pertains to the modeling of geologic volumes of the earth's crust. It further pertains to a system for developing a model of a geologic volume by locating positions of observations within the model which correspond to known positions of observations in the geologic volume, and thereafter extrapolating from or interpolating between such positions of observations. The model is composed of a plurality of small incremental volumetric elements configured to resemble corresponding incremental volumetric elements in the geologic volume. The invention has particular application to the study and modeling of any volume of the earth's crust comprising layered (stratigraphic, bedded) rock, strata, or deposits.
2. Description of the Related Art
The modeling of geologic volumes is widely practiced. Modeling is important in many different industries and fields of technology. It is important, for example, in assessing ground water resources and in plotting the migration of toxic chemicals. Expert witnesses in lawsuits may choose to base their testimony on modeling. Attorneys may choose to use certain models as demonstrative evidence before a judge or jury. Various federal agencies require modeling of certain parts of the earth's crust. It is also important in the mining and petroleum industries to locate minerals. The purpose of such modeling is to organize known information on geologic volumes and to predict the nature and distribution of descriptive attributes and/or quantitative values within the volume, thereby facilitating studies and actions relative to the volumes.
Modeling may be performed in several ways, as for example, by making maps or sections of volumes directly from the information. Generally speaking, a map is a two-dimensional projection on a horizontal planar surface of a representation of features of the volume modeled. A section, on the other hand, is normally a graphic representation of the volume projected on a vertical plane cutting the volume. The present invention has application for producing both maps and sections. Another way to model is to systematically store the information in computers, and thereafter recover the information as desired. Recovery of the stored information in some instances may involve feeding the information to plotters which automatically plot the data in map or section form. In general, then, the art of modeling a geologic volume in a first aspect resides in building a model of the volume by assembling known data as well as extrapolated and interpolated data throughout the modeled volume. Once the model is built, displays such as maps, cross-sections, and statistical information result from the model.
Modeling the earth's crust, including map and section making, involves complex geological and geophysical relationships and many types of data and observations. Of particular interest are geological volumes of sedimentary rocks or deposits, since almost all oil and gas, many mineral deposits, and most ground water normally occur in sedimentary deposits--typically in porous reservoirs such as clastic (sandstones), secreted, and/or precipitated deposits. Such deposits generally exist in layers (strata, beds), formed over periods of geological time by various physical, chemical, and biological processes. The deposits may have been formed by rivers dropping sediments at their deltas, by windblown sediment, by wave and marine action, by tidal action, by precipitation from a solution, by secretions by living organisms, or by other mechanisms. The deposits may have been modified by weathering, erosion, burial, and structural movement.
A present day layer or formation of sedimentary rocks or deposits was originally laid down on a depositional surface (time line) that was either essentially horizontal or at an angle or slope (depositional slope) with respect to a horizontal plane (sea level). The deposited layer may have experienced vast changes in position and configuration with time. Forces of burial, compaction, distortion, lateral and vertical movement, weathering, etc. may have resulted in the formation being fractured, faulted, folded, sheared, or modified substantially. As a result, any given geologic volume will normally be found to be a complex relationship of rock layers which may extend thousands of feet below the surface of the earth to the earth's mantle. A particular geologic volume may involve numerous superimposed layers of sediments, which were originally deposited on a horizontal or sloping depositional surface and may be subsequently tilted, fractured, folded, pierced, overturned, faulted, weathered, eroded, or otherwise modified in many ways.
Of special interest to geologists and groundwater experts interested in oil, gas, minerals, and water are geologic deposits or strata having porosity and permeability which enable them to transport or hold fluids or other materials of economic importance. Geological features such as anticlines, faults, stratigraphic traps and salt domes are of particular importance because of their ability to trap and store such fluids.
A common problem for earth scientists is to reconstruct, interpret, or determine what the "state of nature" of any given geologic volume comprises from: (1) examination of the surface of the earth, (2) examinations of fluids, cores, or well cuttings from penetrations of the earth, or (3) observation of physical, chemical, or biologic response from either penetrations or surface observation. Their attack on the problem has given rise to many specialized fields of geology, geophysics, geochemistry, well logging, etc. Their work has resulted in the development of many types of techniques and apparatus. Application of the techniques and apparatus, in turn has resulted in numerous types of data. Some of the data are related to determining the relative amounts of water, minerals, oil and gas, etc. in a given formation; some are related to describing and identifying characteristics of rocks (lithology) or identifying fossils on the surface or in different wells; and some are related to determining properties such as permeability, porosity, or structural attitude by observing the earth's electric, seismic, or radiologic response.
Geologic data may be obtained by studying outcrops at the surface of the earth. Geophysical surveys, including magnetic, gravity and seismic surveys, also supply data. Drilling oil wells also supplies data. Thus, in the oil industry drill cuttings and drilling fluids are typically analyzed for hydrocarbon content. Other study items in the search for oil include fossils, pollen, pore volumes, nature or facies of rocks, environments of deposition, sand and shale ratios, etc. Various types of logging tools are also run into wells to record well logs of all types, including, e.g., electric logs, radiation logs, and magnetic logs.
Substantial efforts are made in studying any given geologic volume to obtain as much data as possible about the volume. Even though several wells may be drilled, and numerous geophysical surveys made, it will nevertheless be a common practice to interpolate and extrapolate critical data throughout the volume. This is particularly the case for data obtained from laterally spaced wells in the same geologic volume. However, interpretation by manual interpolation and extrapolation of data is tedious, time consuming, and may be subject to errors of logic. It is also difficult, because geologic layers, strata, or beds almost never lie above one another in neat, consistent, horizontal, and laterally extensive sequences. The formations vary in their lateral extent and spatial position and attitude and the interpolation and extrapolation must take this into account.
The advent of digital computers and automatic plotters has been a great aid to the tasks of storing, recovering, and mapping data in simple presentations. And in recent years, efforts have been made to generate or build actual models of geologic volumes. Much, however, remains to be done.
Three-dimensional geologic modeling techniques currently in use rely generally on the use of relatively thin, horizontally disposed layers extending throughout a geologic volume. The bounding surfaces between the layers are smooth and horizontal. Each layer is divided longitudinally and laterally by a grid of lines, so that each layer consists of a plurality of cells which are right-angled hexahedrons or parallelepipeds of uniform size resembling tiles. Each edge of these grid cells or elements is selected to represent a particular distance in the geologic volume being modeled. Generally, the horizontal edges represent much greater distances than do the vertical or corner edges or risers. For example, the horizontal edges or dimensions of each cell may typically represent from tens of feet to miles of an actual volume; and each riser may typically represent from several inches up to several hundred feet or more.
A person using prior art models will assign each cell in the model an address, comprising, e.g., the x, y, and z coordinates of that cell. These coordinates may then be used as an address in storing, processing and recalling the data for that particular cell.
Geologic modeling techniques currently practiced generally make use of all available data on a geologic volume to be modeled. First, the exterior boundaries of the overall geologic volume under study are defined.
The volume is divided along three orthogonal axes into a plurality of vertically superimposed and horizontally disposed layers. In the most sophisticated prior art modeling systems, grids representing geological time horizons are used to control lateral interpolation of geological parameters within columns of cells between these grids. This method of correlating well interval data to cells by ratio of vertical distances within sequences is described below. Although the surfaces represented by these grids may not be horizontal, the practice has been to make the layers in the model volume horizontal. Each layer is then divided into a plurality of cells, such that each layer resembles a grid, and each cell in each grid is part of a vertical column of cells, as well as part of a horizontal row of cells. All the horizontal edges of all the cells are equal in length, and the riser, being the height of every cell, is equal, and is selected to be a suitable measure in relation to the geologic volume in which the cells appear. Each geologic volume of interest, then, is modeled by a plurality of cells arranged in orthogonal grids of rows and vertical columns. Each cell can be systematically identified and handled by reference to its position in the several grids and columns in the three-dimensional model array. Modern computers with their extensive data storage and handling capacities are well suited for this service.
Known properties or parameters or attributes for each cell in a given volume between time horizon grids are then incorporated in the model. Thus, as an example, assuming one well provides data, the values of particular attributes within a well interval that apply to a particular cell are assigned to that cell. Interpolation is then made of the values of the same attributes for each cell in the columns to either side of the well. These interpolations may be made in several ways. As one example, if for a geologic sequence, a column in question is short and has only three vertical cells, and the well supplying data for the same sequence has a much greater vertical length, the well interval is divided into thirds. Then, either the average or the plurality of the values of the attribute and/or values of the top third of the well column are assigned to the top cell of the three-cell column. If the column in question consisted of only two cells, then the plurality or average of the attribute and/or values of the top half of the well interval would be assigned to the top cell of the two-cell column. If the column in question consisted of only one cell, then the plurality or average of the attribute and/or values in the entire well interval would be assigned to the one-cell column. Ultimately, then, it becomes possible to map values of various parameters for a given sequence at one or more given horizontal positions within the sequence.
The above system of modeling geologic volumes has been of some aid to geologists nd other persons who study such volumes. The system is of particular value in searching for and evaluating deposits of oil and gas. Nevertheless, the system has several shortcomings. A principal shortcoming is that the system does not take into account the fact that the present position of layers of a geologic sequence rarely, if ever, lie in a perfectly horizontal orientation. Although, as mentioned earlier, sedimentary layers are normally formed on a depositional surface which is essentially horizontal or on a sloping surface, this condition rarely persists after any substantial period of geologic time.
A stratigraphic sequence in nature is a volume or sub-volume of the earth's crust where depositional surfaces or time-lines (often strata boundaries) and surfaces such as unconformities, diastems, and surfaces of intrusive or diapiric masses (surfaces which define and bound a sequence) form a geometric style or pattern that sets it apart from other sequences; and where the stratigraphy and facies within the sequence may share a similar or common depositional history.
In general, any given natural stratigraphic sequence containing layers of rock may thicken, thin, be horizontal or sloping, or interact with unconformities or diastems to lead to a pattern or style of non-horizontal layers which is complex and varied. After deposition, structural movements with time can place the layers in an almost unlimited variety of attitudes; not simply horizontally stacked layers.
Modeling systems of the prior art, since they have horizontal layers, result in faulty or incomplete interpretations of data. For example, cells at a common depth at two laterally spaced wells are assumed by many prior art modeling systems to be in a common layer, but in reality may be from two different layers separated greatly in geologic time. Therefore, it often happens that interpreting data based on the assumption of horizontal layers results in an inaccurate interpretation.
The prior art modeling systems also fail to adequately take into account the different stratigraphic and structural configurations of layers which exist in nature. Varying configurations of stratigraphic patterns or styles are all treated alike in that the same basic type of modeling cell, and the same cell boundary surfaces, are used to model every pattern of stratigraphy within a geologic volume, even though the stratigraphic pattern or style within sequences may be varied and substantially different.
In summary, prior modeling processes have horizontal layers of cells of equal size, and with prior processes, data is interpreted into the cells by comparison and ratios. This all leads to inaccurate modeling of layered rocks in the earth's crust.